In order to limit the pollutant emissions, modern motor vehicles are equipped with a tank venting apparatus, in which the fuel vapors escaping from the fuel tank are stored. An integral part of the tank venting apparatus is a fuel vapor reservoir, in which the fuel vapors are adsorbed. Once the load limit of the fuel vapor reservoir is reached, the latter must be regenerated by a tank venting operation. For this purpose the fuel vapor reservoir is connected to the inlet manifold of the internal combustion engine by way of a venting line. Arranged in the venting line is a controllable tank venting valve, which is opened for performing the tank venting operation, so that the fuel vapors adsorbed in the fuel vapor reservoir escape into the inlet manifold due to the negative pressure in the latter, and take part in the combustion. Accordingly a tank venting operation is possible only when a negative pressure prevails in the inlet manifold relative to the tank venting apparatus. In the case of supercharged internal combustion engines, in the supercharged operating state a higher pressure prevails in the inlet manifold than in the tank venting apparatus. In order to protect the tank venting valve, therefore, a safety valve in the form of a non-return valve is often arranged in the venting line between the tank venting valve and the inlet manifold. This safety valve prevents any gas flow from the inlet manifold in the direction of the tank venting valve and thereby protects the tank venting valve against excessive pressure loading.
Further reaching, statutory provisions prescribe testing of the tank venting apparatus for leaks. One known method, natural vacuum leak detection (NVLD), therefore exploits the natural formation of a negative pressure in the tank venting apparatus during cooling, in order to test the leak tightness. For this purpose the tank venting apparatus usually comprises either a pressure sensor or a pressure switch, which indicate the pressure level in the tank venting apparatus. The working of this pressure switch must be tested in accordance with statutory provisions.